Recently, high density polyethylene has achieved commercial success in such products as films, pipes and blow moulding materials. Among these new polymers are high molecular weight high density polyethylene resins having a broad molecular weight distribution.
Polyethylene resins manufactured by using modern high productivity catalysts have often relatively narrow molecular weight distribution. These resins are not ideal for the manufacture of high density polyethylene for extrusion, e.g., film, pipe or molding resins.
Bimodal polyethylenes are polymers, which are composed of low and high molecular weight fractions. These polymers are designed to have a certain balance between fair processability and good mechanical strength properties. The low molecular weight fractions improve the processability properties of the polymer and the high molecular weight fractions improve mechanical properties. In order to increase strength properties the proportion of high molecular weight fractions could be increased. The result is, however, that the product becomes more difficult to process, for example in film extruders. The processing consumes more energy and the surface quality of the films or pipes tend to be inferior. In other words the processability of the product sets certain limits to the proportion of the high molecular weight component. On the other hand the processability can be improved by using as the first component a polymer having very low molecular weight.
In practise it has been found that in view of product properties the low molecular weight component has preferably the following properties: a weight average molecular weight (M.sub.w) of 5,000-25,000, a ratio between the weight average molecular weight and the number average molecular weight (M.sub.n) of 2.5-9 and a density of 0.960-0.980 g/cm.sup.3 for bimodal high density polymer and a 0.925-0.940 g/cm.sup.3 for bimodal low density polymer. The melt index (MFR.sub.2) of this fraction is typically within the range of 10-1000 g/10 min. The molecular weight of the bimodal end product is between 150,000-400,000, typically 200,000-350,000 and the ratio M.sub.w /M.sub.n is between 20-40. By calculating from these values or experimentally it is obtained for the high molecular weight component typical M.sub.w -values of about 400,000-900,000, M.sub.w /M.sub.n -values of 4.5-9.5 and densities for bimodal high density materials of 0.930-0.960 g/cm.sup.3 and for bimodal low density materials 0.900-0.920 g/cm.sup.3. The proportions between the low molecular weight component and the high molecular weight component vary typically between 40:60-65:35 for bimodal polyethylene resins.
According to prior art polyethylene resins having a broad molecular weight distribution are manufactured by two principal methods. The first method consists of blending two or more unimodal polymers having a different molecular weight. The first component can have a greater proportion of relatively low molecular weight fractions and the further components can have a greater proportion of high molecular fractions. Such method is disclosed for example in European patent No. 0 100 843, in which polyethylene is manufactured by blending together 60-30 parts by weight of a low molecular weight high density ethylene polymer having a melt index (MI) within the range of 45-300 g/10 minutes and a density of 0.950-0.975 g/cm.sup.3, and 40-70 parts by weight of a high molecular weight high density ethylene polymer having a melt index (MFR.sub.21) within the range of 0.1-1.5 g/10 min and a density within the range of 0.930-0.945 g/cm.sup.3. Another example of blending two unimodal polyethylene components together is EP533160. Also it is known to blend two or more unimodal polyethylene components having different ranges of molecular weights (see EP129312) or blend an unimodal polyethylene component and a bimodal polyethylene component (see EP517222).
The second basic method for producing ethylene polymers having a broad molecular weight distribution consists of a multiphase polymerization process, in which in one reactor a polymer having a low molecular weight and a narrow molecular weight distribution is prepared. Another polymerization reactor is run under conditions, which favor the formation of polymers having a high molecular weight. The reactors are in series so that the polymerization started in the first reactor continues in the second one. In this way it is possible to obtain bimodal ethylene polymers. Also it is known to blend unimodal polyethylene components and bimodal components prepared by a multi-phase process (see EP129312).
Both methods above have certain drawbacks. In the blending method it has been found that the gel content and the number of the fish-eys in the blended product tend to be high. Another drawback in the method is that the possibilities to grade changes are very limited. The only possibility to affect the product properties is to vary the proportions of the blend components, which can be made only within certain limits. Very often a grade change requires selection of components having different molecular weights and/or different densities. This means that the producer has to keep several different blending components available, which demands great investments to storage facilities.
In the continuous multiphase polymerization method the process control is more complicated than in the single-phase polymerization. The transition times are long, especially when the conditions in the first reactor must be adjusted.
In the continuous multiphase polymerization method there are also some drawbacks in the product homogenity. Batch-polymerized bimodal resins can be gel-free, because all particles have the same residence time and composition. A continuous process to produce a bimodal resin in back-mixed reactors gives a mixture of particles with a composition ranging from pure low molecular weight materials to pure high molecular weight materials.
Because of the residence time distribution a part of the material flows immediately through the first reactor forming in the second reactor a lot of high molecular weight fraction. The composition distribution between the particles can be affected either by narrowing the residence time distribution in the first reactor, or removing the small unreacted or less reacted catalyst particles from the polymer before it is fed to the second reactor.
Resins with good mechanical properties can be achieved when in the first reactor a higher proportion of low molecular weight material and in the second reactor a material of very high molecular weight is produced. However the homogenity of the end product will decrease drastically because of composition distribution between particles. Products having a reactor split higher than 55:45 have generally high gel and fish-eye content in the end product.
The expression "reactor split" used in this application means the ratio of the low molecular weight material to the high molecular weight material in a reactor product. The expression "product split" means the corresponding ratio in the end product.
Without limiting to any specific theory one possible explanation is that if more low molecular weight material (by volume) is produced than high molecular weight material, the shear forces during melt homogenization will not be conveyed easily to the areas of high molecular weight. The high molecular weight particles just tend to float around in the low molecular weight/low viscosity fraction and show up as gels or white spots in the end product.
The inhomogenities can be avoided if lower reactor splits, for example 45:55 is used. However, this is not favorable for combination of good processability and good mechanical performance. To achieve bimodal polymers having reasonably good processing properties, e.g., for film or pipe grade polymers, it is necessary to increase the reactor split, which leads to the drawbacks mentioned above.